1. Field of the Invention
The present invention relates to valves for controlling fluid flow, and more particularly to a needle valve having particular application when low flow rates are desired, as for example when parenterally administering fluids to a patient.
2. The Prior Art
Parenteral administration of fluids to a patient has long been known in the medical art. A conventional parenteral administration system typically includes a container of liquid, an elongated flexible tube and a cannula or catheter that is inserted into the cardiovascular system of the patient. The liquid thus administered may be whole blood, plasma, or any one of a variety of medications.
In the past, infusion of parenteral fluids into a patient has been controlled by partially collapsing a portion of the delivery tube. This may be done with the use of a roller clamp or a pinch-type clamp. The rate of flow is then determined by counting the number of drops per unit of time that occur in a drip chamber.
This technique has proved satisfactory in most cases where desired flow rates are comparatively fast or where the accuracy of the flow rate is not particularly critical. However, there are some instances where greater control of the rate of infusion is of utmost importance. For example, when medication and fluids are administered to patients suffering from renal disorders, very low flow rates must be used and careful control of the flow rate is highly important.
Another example of an instance requiring more carefully controlled infusion at low flow rates includes the parenteral administration of medications by injecting the medication into an infusion fluid and then administering the infusion fluid over an extended period of time. Constant administration of parenteral fluids at very low flow rates has also been used to keep intravenous cannulas and catheters from becoming clogged during central arterial pressure monitoring.
The use of needle valves for obtaining more controlled flow of parenteral fluids is known in the art. Typically, prior art needle valves include a metal housing that is carefully machined to conform to an elongated, tapered needle that fits within the housing. A metal collar is also typically machined and fits onto the housing. The needle screws into the collar and the position of the needle may be adjusted by screwing the needle. Typically some type of gasket is placed over the needle to prevent leakage through the housing.
In the use of this type of needle valve it has been observed that high precision control of fluid flow at very low rates is not possible to achieve because of the difficulty of precisely machining the housing and tapered end of the needle in such a way that they will fit together with a very close tolerance.
More recent types of needle valves have been constructed by separately molding either the housing or the tapered needle and then assembling these components in the same manner as described above. Needle valves constructed in accordance with this method have also proved to exhibit the same difficulties when attempting to achieve precise control of fluids at very low flow rates.
In addition to the inaccuracies inherent with these kinds of devices, they are also relatively expensive to build because they require very detailed machining of metal parts. Although the cost is somewhat reduced with those types of devices which use some parts molded from plastic, the cost of producing the valves is still relatively high because they require additional labor and time to assemble all of the parts.
Accordingly, what is needed is a needle valve that is capable of achieving very precise control of fluid flow at low rates and which can be inexpensively manufactured.